1. Field of the Invention
The present invention generally relates to a pedestal for supporting a substrate, particularly to a heating/cooling pedestal for semiconductor-processing apparatus.
2. Description of the Related Art
During a process of deposition by plasma enhanced chemical vapor deposition (PECVD) or a process of curing by UV irradiation, the temperature of a substrate increases due to the chemical reaction inside a chamber. However, this temperature increase causes a thermal budget problem and also affects film quality. Therefore, the process conditions are required to be set in narrow ranges to inhibit unwanted increases of the substrate temperature.
In order to suppress increases of the substrate temperature, removing excess heat is necessary. There are several methods reported to control heat transfer in heat transfer assemblies such as a pedestal for supporting a substrate including both a heating element and a cooling element which removes excess heat by circulating a cooling medium such as a gaseous or liquid coolant inside the pedestal or between the substrate and the pedestal (e.g., U.S. Pat. No. 7,327,948, U.S. Pat. No. 6,951,587). When the substrate temperature increases beyond a set range during processes, the heating element is turned off, and the cooling element is activated to remove excess heat, thereby controlling the temperature.
However, while the coolant is supplied from the supply side and returned from the return side of the cooling element to absorb excess heat, the temperature of the coolant itself increases. As a result, a great temperature difference in the circulating coolant occurs between the supply side and the return side of the cooling element, wherein the temperature at the return side is significantly higher than that at the supply side. Since the reverse side of the substrate is required to be in contact with or close to the cooling element for effective and stable heat transfer between the substrate and the cooling element and for realizing fine temperature tuning, the temperature of the substrate varies according to the temperature variation of the cooling element, and thus, it becomes difficult to control an in-plane temperature gradient and keep the in-plane temperature uniform. Further, when the cooling element is disposed close to the substrate in order to increase heat transfer, a great temperature difference occurs inside the pedestal, thereby creating a localized thermal expansion difference and causing deformation and damage to the pedestal.
Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made.